Well, first, there are no levels. People accrue skills and attributes as they grow and gain power, but just about no character that my players have come up with has been able to overcome a battleship or a thousand man army by hand. This doesn't mean it can't happen, just that it'd be very hard, very time consuming, and require a lot of dedicated play.
Second, a sword is pointy no matter who is wielding it. Absolutely any character can get lucky enough, or have a good enough strategy, to kill a VIP character. Now, the odds of that happening may border on the absurd sometimes, but that still happens once in a while. I had a player one-shot a Chimaera with "Sandblast" a basic-level earth spell that's only supposed to deal 20 damage, max, out of the Chimaera's 300+ Health. He just kept rolling 10s on his dice. BTW, I only use d10s for this game, and if you roll a 10 on any die roll, you take that 10 and then add the next roll. If you get another 10, it accumulates, potentially forever. OTOH, a 1 is an automatic failure, no matter your skill level. So, if Boss rolls a 1 on his Dodge, and n00b rolls a 10, 10, 10, 4 on his Accuracy, n00b will probably hit Boss. You can extrapolate from there.
Third, most players fall into the trap of overspecializing. It's very easy to do. In fact, to become a master at anything, you really have to spend all your time doing that and almost nothing else. So, a jack-of-all-trades may hit that weak spot on an overspecialized character very easily.
Fourth, all players have an elemental weakness. Yes, it's Elemental Rock-Paper-Scissors from tvtropes. I'll spare you the link. So, Überwarrior may be decked out in Orichalcum Plate Mail, wielding a Flaming Greatsword of Doom, and still get taken down by a student wizard because he knows an Intermediate Water spell.
Lots of ways to keep the playing field even.